.NET: Unhelpful error message for same-table SelectMany in LINQ

The following LINQ is not supported (RegionToRole is a collection, which is not understood by SQL):

users.Where(w => w.Value.ID == userId && !w.Value.IsDeleted)
                .SelectMany(sm => sm.Value.RegionToRole)

However, LINQ engine proceeds to generate an invalid query, which fails later in SQL engine:

Failed to parse query: select _T0._key, _T0._val from "User".User as _T0 , "User".User as _T0 where  ...

We should detect invalid LINQ early and throw an error with explanation.